Feed forward RF power amplifier with high efficiency main amplifier and highly linear error amplifier

ABSTRACT

A feed forward RF power amplifier which provides both high efficiency and minimal distortion in broad bandwidth RF applications. The feed forward power amplifier includes a main amplifier biased to provide high efficiency and an error amplifier biased to provide highly linear operation through substantially the entire operating range. Signal peaks which introduce distortion components at the main amplifier output are cancelled by the linear operation of the error amplifier.

RELATED APPLICATION INFORMATION

[0001] The present application claims priority under 35 USC 119 (e) ofprovisional application serial No. 60/357,496 filed Feb. 14, 2002, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates in general to wirelesscommunication systems and methods and RF power amplifiers and methods.

[0004] 2. Description of the Prior Art and Related Information

[0005] The two primary goals of RF power amplifier design are linearityover the range of power operation and efficiency. Linearity is simplythe ability to amplify without distortion while efficiency is theability to convert DC to RF energy with minimal wasted power and heatgeneration. Both these requirements are critical for modern wirelesscommunication systems but it is increasingly difficult to provide both.This is due primarily to the bandwidth requirements of modern wirelesscommunication systems which are placing increasing demands on amplifierlinearity. As a practical matter the only way to provide the desiredlinearity has been to employ very large amplifiers operating in a lowefficiency point of their operating range where they are more linear.

[0006] More specifically, linearization of RF power amplifiers isinherently difficult to achieve as RF power amplifiers use a largenumber of non-linear devices which become more and more nonlinear atincreasingly higher output power levels. In practice, high power RFdevices will generate substantial unwanted InterModulation Distortion(IMD) products which appear as spurious signals at the output of the RFpower amplifier. Depending on the input signal type at the input of thepower amplifier these unwanted signals may appear as spectral regrowtharound the base of the wideband signal, e.g., in a CDMA (Code DivisionMultiple Access) system, or as additional carriers if more than onesignal carrier is applied at the input of the amplifier. In general,wireless service providers around the world are subject to manygovernmental rules and regulations, which mandate very strict bandwidthusage. Spectrum constraints, as well as increasing output power levelsdrive performance requirements for RF power amplifiers. To meet theserequirements large amplifiers operated in a highly linear but relativelyinefficient point in their operating range have been used.

[0007] In addition to limiting IMDs in response to regulatory and signalquality requirements, RF power amplifiers employed in modern wirelesscommunication systems must be efficient, i.e., must achieve good DC toRF conversion efficiencies, to avoid unnecessary power dissipation andheat. This requirement is increasing in importance driven by shrinkingvolumes of deployment facilities (e.g. cellular base stations), as wellas reduction in heat exchanger size (or smaller air conditioning units),and reduction in operating noise levels due to cooling fans, as well asother factors. This need for efficiency clearly runs counter to theabove noted need for large amplifiers operated at a linear butinefficient operating point to achieve desired linearity.

[0008] Although there are many different approaches to achieving higherlinearity and good efficiency in RF power amplifiers, feed forwardamplifiers provide a common approach. In feed forward RF poweramplifiers an error amplifier is employed to amplify only IMD productswhich are then combined with the main amplifier output to cancel themain amplifier IMDs. FIG. 1 illustrates a conventional feed forwardamplifier design having a main amplifier 1 and an error amplifier 2. Thebasic elements also include delays 3, 4 in the main and error path,respectively, and main to error path couplers 5, 6, 7 and 8. Additionalelements not shown are also typically present in a conventional feedforward architecture as is well known to those skilled in the art. Thedelays, couplers and error amplifier are designed to inject out of phaseIMDs from the error path into the main amplifier output at coupler 8 tosubstantially eliminate the IMDs in the main amplifier path. Typicallythe main amplifier size in a feed forward system is chosen to be bigenough to handle all or most signal peaks and the error amplifier sizeis relatively small as schematically illustrated in FIG. 1. For example,the error amplifier in conventional feed forward power amplifiers istypically about one ninth the size of the main amplifier. The averageerror amplifier power dissipation is thus quite small. Nonetheless, themain amplifier is large and its efficiency is quite low and thus theoverall feed forward amplifier efficiency is quite low. An obvious wayto improve main amplifier efficiency is to employ a smaller amplifierand drive it harder, however, this introduces unacceptably large IMDsfor modern high bandwidth applications.

[0009] Therefore, a need presently exists for an RF power amplifierdesign which provides both high efficiency and minimal distortion inbroad bandwidth RF applications.

SUMMARY OF THE INVENTION

[0010] The present invention provides a feed forward RF power amplifierdesign which provides both high efficiency and minimal distortion inbroad bandwidth RF applications.

[0011] In a first aspect the present invention provides a feed forwardamplifier comprising an RF input for receiving an RF signal and a mainamplifier receiving and amplifying the RF signal, wherein the mainamplifier is biased in a first bias class of operation. The feed forwardamplifier also comprises a main amplifier output sampling coupler, afirst delay coupled to the RF input and providing a delayed RF signaland a carrier cancellation combiner coupling the delayed RF signal tothe sampled output from the main amplifier. The feed forward amplifierfurther comprises an error amplifier receiving and amplifying the outputof the carrier cancellation combiner. The error amplifier is biased in asecond bias class of operation with higher linearity than the first biasclass. A second delay is coupled to the output of the main amplifier andan error injection coupler combines the output from the error amplifierand the delayed main amplifier output from the second delay so as tocancel distortion introduced by the main amplifier. An RF output iscoupled to the error injection coupler output and provides an amplifiedRF output.

[0012] In a preferred embodiment the ratio of main amplifier to erroramplifier size is from 2 to 1 or from 1 to 2. The first bias class ofoperation is preferably class C or class AB2 and the second bias classof operation is preferably class A or class AB1. The main amplifier maycomprise one or more semiconductor amplifier devices, for example,plural LDMOS amplifier devices, and the device bias current in the firstbias class of operation is preferably between 0 and 0.17 percent ofdevice saturation current or between 1.25 and 2.50 percent of devicesaturation current. The error amplifier may also comprise one or moresemiconductor amplifier devices, for example, plural LDMOS amplifierdevices, and the device bias current in the second bias class ofoperation is preferably between 3.33 and 10.00 percent of devicesaturation current or between 10.00 and 25.00 percent of devicesaturation current. The feed forward amplifier may also further comprisea pre-distortion circuit coupled to the input of the main amplifier anda controller for controlling the operation of the pre-distortion circuitto minimize distortion at the amplifier RF output.

[0013] In another aspect the present invention provides a feed forwardamplifier, comprising an RF input for receiving an RF input signal, theRF input signal having an average operating amplitude range andintermittent signal peaks in a peak range exceeding the averageoperating range. For example, the RF input signal may comprise a spreadspectrum signal, such as a CDMA signal or a WCDMA signal, havingrandomly occurring signal peaks which comprise the peak signal range.The feed forward amplifier includes a main amplifier receiving andamplifying the RF input signal, the main amplifier having a firsttransfer characteristic over its range of operation. The first transfercharacteristic has a substantially linear portion corresponding to theaverage operating amplitude range of the RF input signal and a nonlinearportion corresponding to the RF input signal peak range. The feedforward amplifier also includes a main amplifier output signal sampler,an error path delay circuit coupled to the RF input and providing adelayed RF input signal, and a first cancellation combiner coupling thedelayed RF input signal to the sampled output from the main amplifier.The feed forward amplifier further includes an error amplifier foramplifying the output of the first cancellation combiner. The erroramplifier has a second transfer characteristic over its range ofoperation, the second transfer characteristic having a linear portioncorresponding to substantially all of the average and peak operatingamplitude range of the RF input signal. The feed forward amplifierfurther includes a main path delay circuit coupled to the output of themain amplifier, a second cancellation combiner combining the output fromthe error amplifier and the output of the main path delay circuit so asto cancel distortion introduced by the main amplifier, and an RF outputcoupled to the second cancellation combiner and providing an amplifiedRF output.

[0014] In one specific implementation the range of operation of theerror amplifier may be about 30 dB. The average power range of operationof the error amplifier may be about 10 dB. The power vs gain transfercharacteristic of the error amplifier is preferably linear to less than0.5 dB of gain through about 25 dB or more of the 30 dB operating range.Alternatively, the power vs gain transfer characteristic of the erroramplifier may be linear up to about −4 to −5 dB from peak device power.The range of operation of the main amplifier may be from about −20 dBfrom peak power to peak power and the range of operation of the erroramplifier may be from about −30 dB from peak power to peak power. Thefeed forward amplifier may further comprise a pre-distortion circuitcoupled to the input of the main amplifier and a controller forcontrolling the operation of the pre-distortion circuit to minimizedistortion at the amplifier output. The feed forward amplifier mayfurther comprise a pilot signal generator providing a pilot signal tothe input of the main amplifier and a pilot signal detector coupled tothe amplifier output and the controller.

[0015] In another aspect the present invention provides a method foramplifying a broad bandwidth RF input signal. The method comprisesreceiving an RF input signal having an average operating amplitude rangeand intermittent signal peaks in a peak signal range exceeding theaverage operating range. For example, the RF input signal may comprise aspread spectrum signal having randomly occurring signal peaks whichcomprise the peak signal range. The method comprises amplifying the RFinput signal employing a main amplifier having a first transfercharacteristic over its range of operation, the first transfercharacteristic having a substantially linear portion corresponding tothe average operating amplitude range of the RF input signal and anonlinear portion corresponding to the RF input signal peak range. Themethod further comprises sampling the main amplifier output, delayingthe RF input signal and providing a delayed RF input signal, andcoupling the delayed RF input signal to the sampled output from the mainamplifier so as to provide a distortion component of the sampled outputfrom the main amplifier. The method further comprises amplifying thedistortion component employing an error amplifier having a secondtransfer characteristic over its range of operation, the second transfercharacteristic having a linear portion corresponding to substantiallyall of the average and peak operating amplitude range of the RF input.The method further comprises delaying the output of the main amplifier,combining the amplified distortion component and the delayed output ofthe main amplifier so as to cancel distortion introduced by the mainamplifier and providing an amplified RF output.

[0016] In a preferred embodiment, the range of operation of the erroramplifier may be about 30 dB. The average power range of operation ofthe error amplifier may be about 10 dB. The power vs gain transfercharacteristic of the error amplifier is preferably linear to less than0.5 dB of gain through about 25 dB or more of the 30 dB operating range.Alternatively, the power vs gain transfer characteristic of the erroramplifier may be linear up to about −4 to −5 dB from peak device power.The range of operation of the main amplifier may be from about −20 dBfrom peak power to peak power and the range of operation of the erroramplifier may be from about −30 dB from peak power to peak power. Themethod may further comprise pre-distorting the RF input signal prior toamplifying by the main amplifier.

[0017] Further aspects of the invention will be appreciated from thefollowing detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram of a prior art feed forward poweramplifier.

[0019]FIG. 2 is a block diagram of a preferred embodiment of a feedforward power amplifier in accordance with the invention.

[0020]FIG. 3 is a plot of the transfer characteristic of the mainamplifier in the feed forward power amplifier of FIG. 2 illustrating thegain characteristic over the operating range.

[0021]FIG. 4 is another plot of the transfer characteristic of the mainamplifier in the feed forward power amplifier of FIG. 2 illustrating thephase response over the operating range.

[0022]FIG. 5 is a plot of the transfer characteristic of the erroramplifier in the feed forward power amplifier of FIG. 2 illustrating thegain characteristic over the operating range.

[0023]FIG. 6 is another plot of the transfer characteristic of the erroramplifier in the feed forward power amplifier of FIG. 2 illustrating thephase response over the operating range.

[0024]FIG. 7 is a detailed schematic drawing of one preferredimplementation of a feed forward power amplifier in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] A preferred embodiment of the invention is illustrated in FIGS.2-6. Referring first to FIG. 2 the forward amplifier 10 is illustratedin a block schematic drawing. The feed forward amplifier 10 includes aninput 12 which receives an input RF signal to be amplified and an output14 which outputs the amplified RF signal. The RF signal may be a highbandwidth signal such as a CDMA (Code Division Multiple Access) spreadspectrum communication signal or WCDMA (Wide Code Division MultipleAccess) or other high bandwidth signal. In spread spectrum cellularsystems such as CDMA or WCDMA a number of individual channels or usersare combined and spread over a frequency spectrum by multiplying userdata with a spreading code and then combining the channels. Thespreading code is typically chosen to spread the data from an individualchannel across a relatively wide frequency spectrum, within of coursethe spectrum range available to the given cellular provider. Since manyindividual channels are combined, the peak power of the overall signalprovided to the amplifier 10 will depend on the individual amplitudes ofthe symbols being combined. It is statistically possible that theindividual channel symbols will add to create very large combined symbolpeaks. Although statistically not common, such very large symbol peaksmust be accommodated in the overall system design. In the amplifier 10these signal peaks are accommodated in an efficient manner by exploitingtheir relatively infrequent statistical nature, as will be discussed indetail below.

[0026] Still referring to FIG. 2, the input RF signal is split into amain amplifier signal path and an error amplifier signal path at inputcoupler 30 in accordance with well known feed forward amplifier design.The main amplifier signal path includes main amplifier 16 which isbiased in a high efficiency mode of operation via bias network 18, asillustrated. More specifically, main amplifier 16 is biased at anoperating power level and bias class so that good DC to RF conversionefficiency is provided and wasted power and heat are minimized. As aresult, however, at least some of the signal peaks of the RF inputsignal will enter the nonlinear operating region of the amplifiertransfer characteristic and the main amplifier will operate in aclipping mode introducing distortion for these signal peaks. Theseoperating characteristics of the main amplifier 16 and the manner inwhich this distortion is handled by the error signal path will bediscussed in more detail below.

[0027] The main amplifier signal path further includes input andpre-distortion circuitry 20. The input circuitry may include apreamplifier, group delay circuitry, and gain and phase controlcircuitry generally in accordance with conventional feed forward design.The pre-distortion circuitry in turn pre-distorts the input signal toreduce IMDs introduced by main amplifier 16. Although the pre-distortioncircuitry 20 may be conventional in general design and operation, incombination with the main biasing it allows the main amplifier 16 to beoperated even further into its nonlinear regime while controlling theamount of distortion. A pilot signal source 22 provides a pilot signalwhich is injected into the main amplifier input as illustrated and isused to control the input and pre-distortion circuitry 20. Inparticular, the pilot signal is extracted at the amplifier output bypilot sampling coupler 25 and used by controller 24 to control the inputand pre-distortion circuitry 20 to minimize the pilot signal in theoutput signal and thereby minimize distortion in the output signal. Themain amplifier signal path further includes a main amplifier outputsample coupler 26 and delay 28, generally in accordance withconventional feed forward design. Additional details of the mainamplifier signal path will be described below in relation to a detailedimplementation illustrated in FIG. 7.

[0028] Still referring to FIG. 2, the error amplifier signal pathincludes input signal coupler 30 which samples the RF input signal andprovides it to the error amplifier 34 via delay 32, attenuator/combiner36 and pre-error input circuitry 38. More specifically, delay 32 andattenuator/combiner 36 operate as in a conventional feed forwardamplifier such that the sampled output of the main amplifier 16 isattenuated and combined with the delayed input signal atattenuator/combiner 36 to substantially cancel all but the distortioncomponent of the sampled signal from the main signal path. In someapplications and implementations it may be advantageous to control thecancellation at attenuator/combiner 36 to retain some RF carriercomponent in the resulting signal and the resulting signal is not purelythe distortion component of the main amplifier. Nonetheless, for thepurposes of the present application the resulting signal will bereferred to as the distortion component and it should be understood somecarrier component may be included. This distortion component of thesignal is provided to pre-error input circuitry 38. Pre-error inputcircuitry may include a preamplifier, group delay circuitry, and gainand phase control circuitry which operates similarly to circuitry 20.However, unlike circuitry 20 a pre-distortion circuit is not required inthe error path due to the highly linear nature of the error amplifier.

[0029] The output of circuitry 38 is provided to error amplifier 34which restores the magnitude of the sampled distortion components (IMDs)to that in the main signal path. Error amplifier 34 includes biasnetwork 40 which controls the operating power range and bias class oferror amplifier 34 so that it operates in a highly linear portion of itstransfer characteristic. Since signal peaks in the input RF signal willcreate relatively large peak IMDs sampled from the main amplifieroutput, the error amplifier must be significantly larger thanconventional feed forward error amplifiers. For example, the erroramplifier may range from about one half the size of main amplifier 16 totwice the size of main amplifier 16. This is in contrast to conventionalerror amplifiers which are typically about one ninth the size of themain amplifier. Also, the error amplifier must be biased in a bias classwhich is of high linearity to ensure the operation is linear in theoperating portion of the transfer characteristic. Since the bias classof the error amplifier is high, the error amplifier is inherentlyrelatively inefficient in DC to RF conversion and the error amplifier ispotentially a significant source of wasted power and undesired heat.Nonetheless, in practice it has been determined that the occurrence ofsignal peak related IMDs is sufficiently infrequent that the erroramplifier only acts as a pulse amplifier and its impact on theefficiency of the overall amplifier is minimal.

[0030] Still referring to FIG. 2, the amplified distortion componentoutput from error amplifier 34 is combined with the delayed main signalat 180 degrees (out of phase) with the main amplifier output at errorinjection coupler 42 to cancel the distortion component in the mainsignal path. A substantially distortion free amplified signal is thenprovided to the output 14. Any residual distortion is detected by thepilot detect circuitry 24 and used by the controller to provide controlsignals to circuitry 20 and 38 under the control of controller 24 whichmay be a suitably programmed microcontroller. These two controls may beessentially independent and may be viewed as control of two separateloops; loop 1 comprising circuitry 20, main amplifier 16, main amplifieroutput sample coupler 26, input signal coupler 30, group delay 32 andcombiner 36, and loop 2 comprising sample coupler 26,attenuator/combiner 36, pre-error circuit 38, error amplifier 34, delay28 and error injection coupler 42. More details on the loop controloperation will be provided in relation to a preferred detailedimplementation described in relation to FIG. 7.

[0031] Referring next to FIGS. 3-6 and below Table 1, the operatingcharacteristics of main amplifier 16 and error amplifier 34 will bedescribed in more detail. These figures and Table 1 assume that 100 WattP1dB devices, specifically 100 Watt LDMOS (Laterally Diffused MetalOxide Semiconductor) amplifier devices with saturation at about 1 dB,are used in the main and error amplifiers. Table 1 provides bias classesfor the main amplifier and error amplifier in terms of quiescent biascurrents (Idd) as a percentage of saturation current (Idss) for theamplifier devices. FIGS. 3-6 in turn illustrate the transfercharacteristics for the main and error amplifiers over their operatingrange. Although these specific values correspond to one device example,i.e. 100 watt LDMOS P1dB devices, these bias class characteristics,operating ranges and transfer curves will scale quite generally acrossboth larger and smaller devices. Accordingly, these bias classdefinitions, operating ranges and transfer curves are not limited to thespecific power example. Nonetheless, the bias class definitions of Table1 may not specifically correspond to the Table 1 device parameters forall amplifier device types. The distinction between bias classes and thedefinition of class C, class AB2, class AB1 and class A are generallyunderstood in the art for a wide variety of devices, however, andtherefore the Table 1 device parameter values should be viewed asillustrative and not limiting in nature. TABLE 1 Nominal Quiescent BiasCurrents, per device, at 25° C. 12.0 Amp Idss Main Min Main Max ErrorMin Error Max Class of Idd % Idd % Idd % Idd % Operation (Amp) Idss(Amp) Idss (Amp) Idss (Amp) Idss C 0.000 0.00% 0.020 0.17% AB2 0.1501.25% 0.300 2.50% AB1 0.400  3.33% 1.200 10.00% A 1.200 10.00% 3.00025.00%

[0032] As may be seen from Table 1, the main amplifier is biased in ahigh efficiency class, specifically Class C or lower Class AB (AB2).This provides the desired maximum DC to RF conversion efficiency for agiven device size. In addition to reducing wasted power, this DC to RFefficiency increases reliability. More specifically, when modern RFpower devices such as LDMOS amplifier devices are operated at higherefficiency levels this directly translates into lower junctiontemperature. Reduction in junction temperature greatly increases themean lifetime of the device and thus improves overall reliability of thefeed forward power amplifier system. The transfer curves of FIGS. 3 and4 in turn show that some nonlinearity in the transfer characteristicoccurs throughout the normal or average power operating range of themain amplifier due to the biasing class of the main amplifier(especially in the power vs gain transfer curve). This nonlinearity willintroduce some distortion (IMDs) through the normal operating range.However, as shown in FIG. 3, this nonlinearity is only about 0.5 dB gainor less through this average power region of the transfer curve; i.e.,between P1dB-20 and P1dB-10 in the specific example illustrated.Therefore, this average power region may be characterized as asubstantially linear region of the transfer curve. By the use ofpre-distortion in the main path IMDs due to this nonlinearity can besubstantially eliminated. Therefore, there will be minimal load on theerror amplifier during operation in the average power region.

[0033] The transfer curves of FIGS. 3 and 4 also show that the averagepower region of the main amplifier transfer curve is chosen relativelyclose to the saturation point of the amplifier. Therefore, for a givenpower requirement a smaller device can be used. Since smaller deviceshave lower current and draw less power the DC to RF efficiency isfurther increased. As may be seen from the transfer curves, however,while average signal power levels correspond to operation in thesubstantially linear portion of the transfer characteristic, therandomly occurring peak power signals are in a highly nonlinear portionof the transfer characteristic. This portion of the operating regioncorresponds to significant IMDs in the main amplifier output which aretoo large to be removed by pre-distortion in the main path. As discussedabove, however, these signal peaks are relatively infrequent for typicalhigh bandwidth signals, such as CDMA and WCDMA RF input signals.Therefore, the main amplifier 16 operates in a high efficiencysubstantially linear mode the majority of the time but generatesintermittent large IMD products. These intermittent large IMD productsin turn must be removed from the RF output by the operation of the erroramplifier.

[0034] Referring to Table 1 and FIGS. 5-6, the error amplifier operatingcharacteristics will next be described. Table 1 shows that the biasclass for the error amplifier is significantly higher than for the mainamplifier, e.g., higher AB (AB1) or Class A. Also, the size of the erroramplifier is selected so that the majority of the operating range iswell away from the nonlinear portion of the transfer curve near peakpower. In some demanding applications the error amplifier may be largerthan the main amplifier, up to about double the main amplifier size.Alternatively, in less demanding applications the error amplifier may besmaller than the main amplifier, e.g., about half the main amplifier.This combination of error amplifier size and bias class provides alinear transfer characteristic for the error amplifier oversubstantially all of its operating range, including signal peaks, asshown in FIGS. 5 and 6. This allows the error amplifier to accuratelyamplify peak signal IMDs so as to cancel the main amplifier distortion.Therefore, amplifier 10 provides an RF output substantially free of IMDsthroughout the operating range of the RF input, including signal peaks.Although the bias class and operating range of the error amplifier areselected for maximum linearity and not efficiency, nonetheless theoverall efficiency of the feed forward amplifier will not besignificantly affected since the signal peaks are intermittent innature. That is, the error amplifier effectively acts in a pulse modewith relatively low total power consumption.

[0035] Although the error amplifier transfer curve of FIG. 5 shows alinear response through less than all the operating range, about 25 −26dB of the 30 dB operating range (i.e., to about −4 to −5 dB from peakdevice power taken to be 1 dB) in practice this will catch virtually allof the signal peak IMDs since peaks outside this power range will beextremely rare. Nonetheless, this linear range may be varied somewhatdepending on the statistics of the peak power pulses for the specifictype of RF input signal and the IMD tolerance of the particularapplication. The phase response of FIG. 6 is less critical but again thelinear region may varied with the application. In particular, the linearportion of the operating range of the error amplifier transfer curve maybe increased by increasing the size of the error amplifier therebymoving the entire operating range further to the left in FIGS. 5 and 6.Therefore, it will be appreciated that if needed for a particularapplication the linear portion of the error amplifier transfer curve mayextend through the entire operating range including the entire peaksignal range.

[0036] Accordingly, it will be appreciated that the present inventionprovides a feed forward amplifier with high efficiency and minimalwasted power and unwanted heat generation while at the same timeproviding the high degree of linearity needed for wide bandwidthapplications.

[0037] Referring to FIG. 7, a specific implementation of the feedforward power amplifier of FIG. 2 is illustrated. This specificembodiment corresponds to equal size main and error amplifier modules16, 34, respectively, although the operating characteristics of the twomodules will be very different as discussed in detail above. This mayhave cost advantages due to the use of the same basic amplifier moduleconfiguration, thereby avoiding separate design and tooling costs andcosts associated with separate manufacturing steps for the two modules.Performance requirements may offset the cost advantages in certainapplications, however, and the ratio of main to error amplifier size maybe from one-to-two to two-to-one, as discussed above. Also, the specificimplementation of FIG. 7 shows a specific dual control loopconfiguration including main and error path predistortion, which mayreduce distortion due to signal peaks and improve overall amplifierefficiency.

[0038] More specifically, as illustrated in FIG. 7 the main amplifierpath includes RF input 12, input signal coupler 30, input andpre-distortion circuitry 20, pilot injection coupler 107, main amplifierpower module 16, main amplifier output sample coupler 26, group delayline 28, error injection coupler 42, pilot detect coupler 25, isolator114, and RF output 14. The input and pre-distortion circuitry 20 in theillustrated embodiment comprises distributed small signal gain stages102, group delay adjusting circuit 103, pre-distortion circuits 104,phase control circuit 105, and amplitude control circuit 106.Distributed small signal gain stages 102 may be conventional and operateto provide a small initial gain to the RF input signal. Group delayadjusting circuit 103, pre-distortion circuit 104, phase control circuit105, and amplitude control circuit 106 operate under the control ofcontroller 230 which may be a suitably programmed microcontroller.Controller 230 receives the detected pilot signal from pilot detectcircuit 115 and employs this signal to adjust the group delay adjustingcircuit 103, pre-distortion circuit 104, phase control circuit 105 andamplitude control circuit 106 to minimize the pilot signal and henceminimize distortion. Controller 230 also may provide a signal to pilotgenerator 22 which is used to create the pilot signal. In accordancewith typical feed forward control this action may be best described asone loop of a two loop control process as discussed below. The mainamplifier power module 16 includes main amplifier stages 110, 111, 112,and 113 biased, as discussed in relation to Table 1 above, in class C orAB2. This four stage implementation corresponds to four separatedevices; e.g. a pre-amplifier 110, an intermediate power amplifier 111,and two large power amplifiers 112, 113, e.g. 100 Watt 1 dB mainamplifier LDMOS stages. It will be appreciated that more or fewer stagesmay be employed depending on the application and power requirements andon the available stage amplifiers performance and cost.

[0039] Still referring to FIG. 7, the error amplifier path includesgroup delay line 32, carrier cancellation offset injection coupler 202,carrier cancellation detector 203, carrier cancellation coupler 206,error input signal sample coupler 207, error input signal test port 209,pre-error input 38, error amplifier power module 34, and output erroramplifier interconnection lines 216 to the error injection coupler 42.The carrier cancellation coupler 206 receives the sampled and attenuatedmain amplifier output from main amplifier output sample coupler 26, mainsampled signal attenuator 210 and associated RF interconnections 217.The carrier cancellation detector 203 detects the amount of RF carrierin the error path and provides this as an input to controller 230. Atest signal may be applied to the error path via test port 209. Thepre-error input circuitry 38 comprises distributed small signal gainstages 208, group delay adjuster circuit 211, phase adjustment circuit214 and amplitude adjustment circuit 213. The distributed small signalgain stages 208 may be conventional in operation. The group delayadjuster circuit 211, phase adjustment circuit 214 and amplitudeadjustment circuit 213 operate under the control of controller 230, asdiscussed below. The error signal is provided from pre-error inputcircuitry 38 to error amplifier power module 34. Error amplifier powermodule 34 comprises stages 220, 221, 222, and 223 corresponding to themain module 16 layout in the illustrated equal size main and errorimplementation. These stages 220, 221, 222, and 223 may comprisedevices, e.g. LDMOS amplifier devices, of the same size as in the mainmodule but differently biased. In particular, as discussed above inrelation to Table 1, the stages 220, 221, 222, and 223 may compriseLDMOS amplifiers biased in class AB1 or A. This will provide highlylinear amplification of the error signal for current high bandwidthapplications. If future even higher bandwidth requirements or higherpower applications increase the error signal input the error amplifiersize may be increased to maintain the linearity across the operatingrange, as discussed above in relation to FIGS. 5 and 6. The amplifiederror signal is then applied via error amplifier output interconnectionlines 216 to the error injection coupler 42 where it cancels IMDs in themain path.

[0040] The following discussion of the specific control implementationof FIG. 7 will clarify the use of the controller 24 and circuitry 20 and38 to help achieve desired IMD performance. As discussed above, thiscontrol may be viewed as separate control of two loops in accordancewith conventional feed forward control terminology. The first controlloop (or loop 1) is the carrier cancellation loop in accordance withconventional terminology. Loop 1 contains the following circuitelements:

[0041] Input signal coupler 30

[0042] Distributed small signal gain stages 102

[0043] Main path group delay adjusting circuit 103

[0044] Main path pre-distortion circuit 104

[0045] Main path phase 105 and amplitude 106 control circuits

[0046] Pilot injection coupler 107

[0047] Main amplifier power module 16

[0048] Main amplifier output sample coupler 26

[0049] Associated main output RF interconnections 217

[0050] Sampled signal attenuator 210

[0051] Group delay line 32

[0052] Carrier cancellation offset injection coupler 202

[0053] Carrier cancellation detector 203

[0054] Carrier cancellation coupler 206

[0055] Error input signal sample coupler 207

[0056] Error input signal test port 209.

[0057] Loop 1 carrier cancellation detector 203 behavior is similar to aconventional feed forward power amplifier system. The controller 230adjusts the group delay circuit 103 and phase and amplitude controlcircuits 105, 106 to set the detected carrier signal at carriercancellation circuit 203 to a desired level. Controller 230 may operateso that when the carrier cancellation is adjusted there is a minimumamount of RF energy incident upon the input of the carrier cancellationcircuit 203. As noted above, however, in some applications it may beadvantageous to adjust the carrier cancellation so that some RF carriercomponent remains. Loop 1 pre-distortion circuit 104 is controlled bycontroller 230 by monitoring the pilot signal from pilot detect circuit115 to minimize the detected pilot signal.

[0058] Loop 2 is the error path loop or auxiliary path loop. Loop 2contains the following circuit elements:

[0059] Main amplifier output sample coupler 26

[0060] Loop 2 group delay line 28

[0061] Main sampled signal attenuator 210 and associated RFinterconnections 217

[0062] Carrier cancellation coupler 206

[0063] Error path input signal sample coupler 207

[0064] Distributed small signal gain stages 208

[0065] Group delay adjuster circuit 211

[0066] Phase 214 and amplitude 213 adjustment circuits

[0067] Error amplifier power module 34

[0068] Output Error Amplifier interconnection lines 216

[0069] Error injection coupler 42.

[0070] Loop 2 actions are controlled by operation of controller 230. Asthe case of Loop 1 the detected pilot signal is used to monitor andadjust loop 2 cancellation performance. Those skilled in the art willappreciate details of pilot requirements in order to control loop 2performance and stability of circuits 211, 213 and 214. Pre-distortioncircuit 104 is used under control of controller 230 by minimizing thedetected pilot signal together to improve AM/AM and AM/PM performancedue to higher output power levels. The use of a main path pre-distortioncircuit thus improves IMD to carrier ratio at higher output powerlevels.

[0071] A preferred embodiment of the present invention of an RF poweramplifier design which provides both high efficiency and minimaldistortion in broad bandwidth RF applications has been described inrelation to the various figures. Nonetheless, it will be appreciated bythose skilled in the art that a variety of modifications and additionalembodiments are possible within the teachings of the present invention.For example, a variety of specific feed forward circuit implementationsand loop controller implementations may be provided employing theteachings of the present invention and limitations of space prevent anexhaustive list of all the possible circuit implementations or anenumeration of all possible control implementations. A variety of otherpossible modifications and additional embodiments are also clearlypossible and fall within the scope of the present invention.Accordingly, the described specific embodiments and implementationsshould not be viewed as in any sense limiting in nature and are merelyillustrative of the present invention.

What is claimed is:
 1. A feed forward amplifier, comprising: an RF inputfor receiving an RF signal; a main amplifier receiving and amplifyingsaid RF signal, said main amplifier biased in a first bias class ofoperation; a main amplifier output sampling coupler; a first delaycoupled to the RF input and providing a delayed RF signal; a carriercancellation combiner coupling the delayed RF signal to the sampledoutput from the main amplifier; an error amplifier receiving andamplifying the output of the carrier cancellation combiner, said erroramplifier biased in a second bias class of operation with higherlinearity than said first bias class; a second delay coupled to theoutput of the main amplifier; an error injection coupler combining theoutput from the error amplifier and the delayed main amplifier outputfrom the second delay so as to cancel distortion introduced by the mainamplifier; and an RF output coupled to the error injection coupleroutput and providing an amplified RF output.
 2. A feed forward amplifieras set out in claim 1, wherein the ratio of main amplifier to erroramplifier size is from 2 to
 1. 3. A feed forward amplifier as set out inclaim 1, wherein the ratio of main amplifier to error amplifier size isfrom 1 to
 2. 4. A feed forward amplifier as set out in claim 1, whereinthe first bias class of operation is class C or class AB2.
 5. A feedforward amplifier as set out in claim 1, wherein the main amplifiercomprises one or more semiconductor amplifier devices and wherein thedevice bias current in said first bias class of operation is between 0and 0.17 percent of device saturation current.
 6. A feed forwardamplifier as set out in claim 1, wherein the main amplifier comprisesone or more semiconductor amplifier devices and wherein the device biascurrent in said first bias class of operation is between 1.25 and 2.50percent of device saturation current.
 7. A feed forward amplifier as setout in claim 1, wherein the main amplifier and error amplifier compriseplural LDMOS amplifier devices.
 8. A feed forward amplifier as set outin claim 1, wherein the second bias class of operation is class A orclass AB1.
 9. A feed forward amplifier as set out in claim 1, whereinthe error amplifier comprises one or more semiconductor amplifierdevices and wherein the device bias current in said second bias class ofoperation is between 3.33 and 10.00 percent of device saturationcurrent.
 10. A feed forward amplifier as set out in claim 1, wherein theerror amplifier comprises one or more semiconductor amplifier devicesand wherein the device bias current in said second bias class ofoperation is between 10.00 and 25.00 percent of device saturationcurrent.
 11. A feed forward amplifier as set out in claim 1, furthercomprising a pre-distortion circuit coupled to the input of the mainamplifier.
 12. A feed forward amplifier as set out in claim 11, furthercomprising a controller for controlling the operation of saidpre-distortion circuit to minimize distortion at the amplifier RFoutput.
 13. A feed forward amplifier as set out in claim 12, furthercomprising a pilot signal generator providing a pilot signal to theinput of the main amplifier and a pilot signal detector coupled to theerror injection coupler output and said controller.
 14. A feed forwardamplifier, comprising: an RF input for receiving an RF input signal,said RF input signal having an average operating amplitude range andfurther comprising intermittent signal peaks in a peak range exceedingsaid average operating range; a main amplifier receiving and amplifyingsaid RF input signal, said main amplifier having a first transfercharacteristic over its range of operation, said first transfercharacteristic having a substantially linear portion corresponding tosaid average operating amplitude range of said RF input signal and anonlinear portion corresponding to said RF input signal peak range; amain amplifier output signal sampler; an error path delay circuitcoupled to the RF input and providing a delayed RF input signal; a firstcancellation combiner coupling the delayed RF input signal to thesampled output from the main amplifier; an error amplifier foramplifying the output of the first cancellation combiner, said erroramplifier having a second transfer characteristic over its range ofoperation, said second transfer characteristic having a linear portioncorresponding to substantially all of said average and peak operatingamplitude range of said RF input; a main path delay circuit coupled tothe output of the main amplifier; a second cancellation combinercombining the output from the error amplifier and the output of the mainpath delay circuit so as to cancel distortion introduced by the mainamplifier; and an RF output coupled to the second cancellation combinerand providing an amplified RF output.
 15. A feed forward amplifier asset out in claim 14, wherein said RF input signal comprises a spreadspectrum signal having randomly occurring signal peaks which comprisesaid peak signal range.
 16. A feed forward amplifier as set out in claim15, wherein said RF input signal comprises a CDMA signal.
 17. A feedforward amplifier as set out in claim 15, wherein said RF input signalcomprises a WCDMA signal.
 18. A feed forward amplifier as set out inclaim 14, wherein the range of operation of the error amplifier is about30 dB and the power vs gain transfer characteristic of the erroramplifier is linear to less than 0.5 dB of gain through about 25 dB ormore of the 30 dB operating range.
 19. A feed forward amplifier as setout in claim 14, wherein the power vs gain transfer characteristic ofthe error amplifier is linear up to about −4 to −5 dB from peak devicepower.
 20. A feed forward amplifier as set out in claim 14, wherein theaverage power range of operation of the error amplifier is about 10 dB.21. A feed forward amplifier as set out in claim 14, wherein the rangeof the operation of said main amplifier is from about −20 dB from peakpower to peak power and wherein said range of operation of said erroramplifier is from about −30 dB from peak power to peak power.
 22. A feedforward amplifier as set out in claim 14, further comprising apre-distortion circuit coupled to the input of the main amplifier.
 23. Afeed forward amplifier as set out in claim 14, further comprising acontroller for controlling the operation of said pre-distortion circuitto minimize distortion at the amplifier output.
 24. A feed forwardamplifier as set out in claim 14, further comprising a pilot signalgenerator providing a pilot signal to the input of the main amplifierand a pilot signal detector coupled to the amplifier output and saidcontroller.
 25. A method for amplifying a broad bandwidth RF inputsignal, comprising: receiving an RF input signal, said RF input signalhaving an average operating amplitude range and further comprisingintermittent signal peaks in a peak range exceeding said averageoperating range; amplifying said RF input signal employing a mainamplifier having a first transfer characteristic over its range ofoperation, said first transfer characteristic having a substantiallylinear portion corresponding to said average operating amplitude rangeof said RF input signal and a nonlinear portion corresponding to said RFinput signal peak range; sampling the main amplifier output; delayingthe RF input signal and providing a delayed RF input signal; couplingthe delayed RF input signal to the sampled output from the mainamplifier so as to provide a distortion component of said sampled outputfrom the main amplifier; amplifying the distortion component employingan error amplifier having a second transfer characteristic over itsrange of operation, said second transfer characteristic having a linearportion corresponding to substantially all of said average and peakoperating amplitude range of said RF input; delaying the output of themain amplifier; combining the amplified distortion component and thedelayed output of the main amplifier so as to cancel distortionintroduced by the main amplifier and providing an amplified RF output.26. A method for amplifying a broad bandwidth RF input signal as set outin claim 25, wherein said RF input signal comprises a spread spectrumsignal having randomly occurring signal peaks which comprise said peaksignal range.
 27. A method for amplifying a broad bandwidth RF inputsignal as set out in claim 25, further comprising pre-distorting said RFinput signal prior to amplifying by said main amplifier.
 28. A methodfor amplifying a broad bandwidth RF input signal as set out in claim 25,wherein the range of operation of the error amplifier is about 30 dB andthe power vs gain transfer characteristic of the error amplifier islinear to less than 0.5 dB of gain through about 25 dB or more of the 30dB operating range.
 29. A method for amplifying a broad bandwidth RFinput signal as set out in claim 25, wherein the power vs gain transfercharacteristic of the error amplifier is linear up to about −4 to −5 dBfrom peak device power.
 30. A method for amplifying a broad bandwidth RFinput signal as set out in claim 25, wherein the average power range ofoperation of the error amplifier is about 10 dB.
 31. A method foramplifying a broad bandwidth RF input signal as set out in claim 25,wherein the range of the operation of said main amplifier is from about−20 dB from peak power to peak power and wherein said range of operationof said error amplifier is from about −30 dB from peak power to peakpower.